Eye-tracking system and method therefor

ABSTRACT

A system for tracking eye location is disclosed. Systems in accordance with the present invention include a scanner for sweeping a first optical signal across the surface of an eye, a detector for detecting a second optical signal reflected from the eye, and a detection circuit for determining a maximum intensity in the second optical signal. In operation, the scanner sweeps the first optical signal over the surface of the eye while the detection circuitry determines a plurality of intensity maxima in the second optical signal. The time between the intensity maxima during the sweep is indicative of the location of the cornea within the eye surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/090,705, filed Dec. 11, 2014, entitled “Electro-Thermo-MechanicalOptical Scanner Suitable for Display, Imaging, or Object TrackingApplications,”, U.S. Provisional Application Ser. No. 62/181,276, filedJun. 18, 2015, entitled “Near-Eye Display and Method Therefor,”, andU.S. Provisional Application Ser. No. 62/266,020, filed Dec. 11, 2015,entitled “System for Tracking Eye Position and Method Therefor,”, eachof which is incorporated herein by reference. If there are anycontradictions or inconsistencies in language between this applicationand one or more of the cases that have been incorporated by referencethat might affect the interpretation of the claims in this case, theclaims in this case should be interpreted to be consistent with thelanguage in this case.

FIELD OF THE INVENTION

The present invention relates to object-tracking systems in general,and, more particularly, to eye-tracking systems.

BACKGROUND OF THE INVENTION

The movement of the human eye can reveal a wealth of information aboutthe neural mechanisms of the brain and vision, as well as anindividual's neurological health, ocular health, interests, andstate-of-mind. In addition, the tracking of eye movement can be used toimprove/augment human-computer interaction, enable gaze-basedman-machine interfaces, and enhance how we interact with wearabletechnology. For example, gaze tracking, which relies on eye tracking,enables many augmentative alternative communication (AAC) devices thatimprove the ability for individuals lacking speech capability and/ormotor skills (e.g., amyotrophic lateral sclerosis (ALS) patients orthose with spinal cord injuries) to interact with the world around them.

In recent years, eye-tracking technology has become more sophisticatedand is increasingly being directed toward non-health-related uses, suchas improving advertising effectiveness, determining optimal productplacement, improving package design, augmenting the automotive drivingexperience, gaming and virtual reality (VR) systems, military trainingand effectiveness augmentation, athletic training, and the like. Foradvertising and/or product placement applications, for example, theactivity of a subject's eyes is tracked while target stimuli (e.g., websites, commercials, magazine ads, newspapers, product packages, etc.)are presented. The recorded eye-tracking data is statistically analyzedand graphically rendered to reveal specific visual patterns. Byexamining fixations, saccades, pupil dilation, blinks and a variety ofother behaviors, the effectiveness of a given medium or product can bedetermined.

Helmet-integrated eye-trackers could potentially improve the ability ofa fighter pilot to better control an aircraft and react to high-speedbattle conditions while in flight. Eye tracking in this scenario mayprovide capabilities to enhance interaction between the pilot andvehicle. In addition, post-flight analysis of the pilot's gaze could beuseful for training purposes. Further, in-flight health monitoring ispossible by tracking changes in the geometry of the eye due to swellingcaused by rapid changes in pressure.

Unfortunately, conventional eye-trackers are slow, bulky, invasive,and/or restrictive for the user. This makes them difficult, if notimpossible to use in many of the applications discussed above. Inaddition, conventional systems generally require cameras and imagingprocessing software. As a result, they tend to be expensive, slow, andpower hungry. Furthermore, they typically exhibit significant lagbetween eye movement and measured eye position, which degrades the userexperience in VR applications. It is possible to improve the resolutionand speed of such eye tracking systems but, to date, these improvementshave come at the expense of user mobility and added system cost.

Eye coils mounted directly on the eye have been used to track eyeposition with high accuracy; however, they require operation within amagnetic field, which restricts the applications in which they can beused. Further, since they are placed directly on the eyeball, they canonly be used for limited times due to eye safety and vision impairmentissues.

Electro-oculograms enable the position of the cornea through closedeyelids; however, they are subject to blink artifacts and signal noise.In addition, they are relatively inaccurate. Further, they requireelectrodes to be attached in or near the eye. As a result,electro-oculograms are relatively unattractive in many applications.

Systems for tracking the limbus (i.e., the boundary between the white ofthe eye and the dark iris) have been used for eye tracking as well.Unfortunately, such limbal-tracking systems are cumbersome, difficult toalign and calibrate, and have poor sensitivity unless used at shortrange.

Perhaps the most common eye-tracking systems are video-based systems,such as image-processing systems described in U.S. Pat. No. 8,955,973.In these systems, a picture of the surface of the eye is taken underinfrared (IR) illumination and captured by a two-dimensional imagesensor. The picture reveals the location of the corneal reflection andthe pupil of the eye. Using complex image processing, a vector from thecenter of the corneal reflection to the center of the pupil iscalculated and this vector forms the basis of an estimate of thedirection of the user's gaze. Video-based eye trackers can be usedremotely or worn by the subject.

Unfortunately, such eye-tracking systems are slow, bulky, restrictivefor the user, and expensive. Wearable systems are bulky and heavy,making them quite uncomfortable for extended use. Remote systems requirecareful positioning and alignment, which can be easily disrupted duringoperation. In addition, the reliance on video capture and imageprocessing leads to the need for good lighting conditions. Further,video capture is difficult to perform tracking through spectacles due tofront surface reflections.

As an alternative to video-based systems, some conventional eye-trackingsystems project a grid of structured light onto the surface of the eye.Unfortunately, an image of the eye surface must still be captured andanalyzed via image processing to estimate eye position. As a result,while such systems typically require less computational complexity thanvideo-based eye-trackers, they still demand significant computationaland energy resources.

A low-cost, high resolution, low power, high-speed, robust eye-trackingsystem would, therefore, be a significant advance in the state of theart.

SUMMARY OF THE INVENTION

The present invention enables eye tracking without some of the costs anddisadvantages of eye-tracking systems of the prior art. Furthermore,inventive concepts of the present invention can be employed to enableadditional tracking capabilities, such as head tracking, fingertracking, agile free-space communications links, single-fiber endoscopy,and the like. Embodiments of the present invention are particularly wellsuited for use in applications such as heads-up displays, virtual andaugmented reality systems, weapons targeting systems, automotive cabincontrol, collision avoidance systems, advertising design and placementeffectiveness studies, website design, pay-per-gaze advertising,athletic training programs, health monitoring systems, medical research,optometry, augmentative alternative communications systems, wearablesensor systems, and environmental control systems.

An illustrative embodiment of the present invention is an eye-trackingsystem that is suitable for mounting on the frame of a pair of eyeglasses. The system includes a laser, a two-dimensional scanning mirror,a discrete detector, and a detection circuit, where the two-dimensionalscanning mirror and detector are mounted on opposite sides of one lensof the eye glasses. The laser and mirror are arranged on the eyeglassframe such that they collectively interrogate a scan region on thesurface of an eye of a subject with an optical input signal that ischaracterized by a far-field pattern having a global intensity maximumregion.

In operation, the laser provides the optical input signal to thescanning mirror, which sweeps the input signal over the scan region ofthe eye over which the cornea is located. In the illustrativeembodiment, the scanning mirror first sweeps the input signal back andforth, at a constant rate, along a first linear path aligned with thehorizontal axis of the eye. The cornea reflects the input signal as areflected signal, which is incident on the detector. The detectorprovides an electrical signal whose magnitude represents the averageintensity of the reflected signal impinging on its surface. As the inputsignal sweeps across the eye in the forward direction, a first peak inthe intensity of the reflected signal arises when the input signal isincident on at least a portion of the cornea of the eye at which thecurvature of the cornea directs the reflected signal towards thedetector. As the input signal is swept back across the eye in thereverse direction, a second peak in the intensity of the reflectedsignal arises when the input signal is again incident on the cornea. Bysweeping the input signal across the eye at a constant rate, the timedelay between the first and second peaks can be used to estimate thehorizontal position of the cornea within the scan region of the eyebeing scanned. Once this horizontal position is determined, the scanningmirror scans the input beam back and forth along a second linear pathaligned with the vertical axis of the eye, where the second linear pathis positioned at the horizontal position of the cornea. This back andforth vertical sweep again gives rise to a pair of peaks whose timing isused to estimate the vertical position of the cornea within the eye.

In some embodiments, the two-dimensional position of the cornea withinthe scan region is estimated via scanning the input beam back and forthalong a first linear path aligned with one of the horizontal or verticalaxis of the eye while a hill-climbing control method is used in theorthogonal axis to substantially maximize the peak amplitude of theelectrical signal provided by the detector.

In some embodiments, the input signal is swept within the scan region ina two-dimensional path, such as a circular path, an elliptical path, aLissajous path, a rhodonea path, and the like. In some such embodiments,only peak locations (not intensities) may be used to reveal eyeposition, and only steady-state sinusoidal functions are applied to thescanning mirror.

In some embodiments, the scanning mirror includes an optical elementthat converts the input signal into a light pattern, such a dot orcross, on the eye. In some of these embodiments, the optical element isa diffractive element, such as a Fresnel lens, holographic element,etc., which is formed on the surface of the scanning mirror.

In some embodiments, a calibration procedure is run before the system isused to track eye position. In some embodiments, a calibration procedureis run periodically while the system is used to track eye position.Calibration procedures in accordance with the present invention includemeasuring the position of the scanning mirror while the user looks ateach of one or more spots on a screen at a fixed location.

In some embodiments, the scanning mirror includes one isothermalactuator for controlling the angle of a reflective surface about anaxis. In some embodiments, an isothermal actuator is used to control theangle of the reflective surface about each of a pair of orthogonal axes.

In some embodiments, the position of the scanning mirror is controlledvia pulse-width modulated drive signals.

An embodiment of the present invention is a system for estimating thecorneal vector of an eye, the system comprising: a first sourceoperative for providing a first optical signal, the first optical signalbeing characterized by a far-field pattern having a global intensitymaximum; a first scanner operative for scanning the first optical signalwithin a scan region on the surface of the eye, the scan regionincluding a surface feature of the eye; a first detector that is adiscrete detector, the first detector being operative for providing afirst electrical signal based on a second optical signal that includes aportion of the first optical signal reflected from at least a portion ofthe scan region; and a detection circuit operative for determining atleast one maximum in the first electrical signal.

Another embodiment of the present invention is a system for estimatingthe corneal vector of an eye, the system comprising: a first sourceoperative for providing a first optical signal, the first optical signalbeing characterized by a far-field pattern having a global intensitymaximum; a first scanning mirror operative for scanning the firstoptical signal within a scan region on the surface of the eye; a firstdetector that is a discrete detector, the first detector being operativefor providing a first electrical signal based on the detected intensityof a second optical signal that includes a portion of the first opticalsignal reflected from at least a portion of the scan region; a detectioncircuit operative for determining a first maximum in the firstelectrical signal at a first time and a second maximum in the firstelectrical signal at a second time; and a processor operative forestimating the location of the surface feature within the scan regionbased on the difference between the first time and second time.

Yet another embodiment of the present invention is a method forestimating the corneal vector of an eye, the method comprising: scanninga first optical signal within a scan region on the surface of the eye;receiving a second optical signal at a first detector that is a discretedetector, the second optical signal including a portion of the firstoptical signal that is reflected from at least a portion of the scanregion; generating a first electrical signal based on the second opticalsignal, the first electrical signal being generated by the firstdetector; determining at least one maximum of the second optical signal;estimating the first position based on the at least one maximum; andestimating the corneal vector based on the first location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of an eye-tracking system inaccordance with an illustrative embodiment of the present invention.

FIG. 2A depicts a schematic drawing of an exemplary geometry for system100.

FIG. 2B depicts a schematic drawing of an exemplary scan region 122.

FIG. 3A depicts an example of a beam profile suitable for use in thepresent invention.

FIG. 3B depicts a plot of the amplitude of the electrical signal outputby detector 204 as a function of mirror angle and percentage of the beamwaist of reflected signal 118 occupied by the aperture of detector 204.

FIG. 4 depicts a schematic drawing of a perspective view of a transmitmodule in accordance with the illustrative embodiment.

FIG. 5A depicts a schematic drawing of a scanning mirror in accordancewith the illustrative embodiment.

FIG. 5B depicts a photograph of a scanning mirror analogous to scanningmirror 406.

FIG. 5C depicts an electrical arrangement suitable for driving ascanning mirror that employs isothermal actuators for each axis ofrotation.

FIGS. 6A-D depict photographs of alternative MEMS-based scanning mirrorsin accordance with the present invention.

FIG. 7 depicts a schematic drawing of a detect module in accordance withthe illustrative embodiment.

FIG. 8 depicts an initialization procedure for approximating thelocation of a surface feature on an eye.

FIG. 9 depicts operations of a method suitable for tracking the positionof a surface feature of an eye in accordance with the illustrativeembodiment of the present invention.

FIG. 10 depicts an exemplary timing diagram in accordance with method900.

FIG. 11 depicts operations of a second exemplary method suitable fortracking the position of a surface feature of an eye in accordance withthe present invention.

FIG. 12 depicts operations of a third exemplary method suitable fortracking the position of a surface feature of an eye in accordance withthe present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a schematic drawing of an eye-tracking system inaccordance with an illustrative embodiment of the present invention.System 100 includes transmit module 102, detect module 104, andprocessor 106. Transmit module 102 and detect module 104 are arranged ona rigid support in a fixed orientation relative to one eye of a testsubject. System 100 enables tracking of a surface feature (e.g., cornea124) within a two-dimensional region of an eye during typical testsubject behavior (e.g., reading, viewing a computer screen, watchingtelevision, monitoring a scene, etc.), and estimating the corneal vectorof the eye based on the location of the surface feature. For thepurposes of this Specification, including the appended claims, the“corneal vector” of an eye is defined as the gaze direction of the eye,which is indicated by a vector extending outward perpendicularly fromthe center of the pupil of an eye.

Transmit module 102 is a sub-system for providing an optical signal andscanning it in two-dimensions over scan region 122 of eye 120. Transmitmodule 102 is described in detail below and with respect to FIG. 4.

Detect module 104 is a sub-system for receiving light reflected fromscan region 122, providing an electrical signal based on the intensityof the reflected light, and detecting one or more maxima in theelectrical signal. Detect module 104 is described in detail below andwith respect to FIG. 7.

Processor 106 is a conventional digital processor and controller (e.g.,a microcontroller, etc.) operative for controlling transmit module 102,establishing system timing, and estimating the two-dimensional locationof cornea 124 within scan region 122. In the depicted example, processor106 communicates with transmit module 102 and detect module 104 viawired connections (not shown) to transmit and receive control signals126 and output signal 128. In some embodiments, processor 106communicates with transmit module 102 and detect module 104 wirelessly.In some embodiments, processor 106 is integrated in one of transmitmodule 102 and detect module 104.

In the depicted example, system 100 is mounted on eyeglass frames 108,which includes temples 110, lenses 112, and bridge 114. System 100 ismounted on frames 108 such that transmit module 102 and detect module104 are on opposite sides of central axis 126 of eye 120. Specifically,transmit module 102 is mounted on the frames such that it can scan inputsignal 116 over the full extent of scan region 122 and detect module 104is mounted on the frames such that it can receive a portion of inputsignal 116 reflected from scan region 122 as reflected signal 118.

FIG. 2A depicts a schematic drawing of an exemplary geometry for system100.

It is an aspect of the present invention that there exists aconfiguration of system 100 that gives rise to a unique point on cornea124 that results in a maximum intensity in the reflection of inputsignal 116 at detector 204 of detect module 104, where detector 204 is adiscrete detector. For the purposes of this Specification, including theappended claims, a “discrete detector” is defined as an optoelectronicdevice having no more than four electrically independent detectionregions on a single substrate, where each detection region is operativefor providing one electrical signal whose magnitude is based on theintensity of light incident upon that detection region. Examples ofdiscrete detectors include detectors having only one detection region,split detectors having two detection regions, four-quadrant detectorshaving four detection regions, and position-sensitive detectors. Thedefinition of discrete detector explicitly excludes individual pixels,or groups of pixels, within array devices for collectively providingspatially correlated image information, such as focal-plane arrays,image sensors, and the like. When input signal 116 is aligned with thispoint, the angular positions of scanner 202 within transmit module 102are indicative of the location of this point of maximum reflectionwithin scan region 122, which is indicative of the corneal vector forthe eye.

FIG. 2A depicts the position of cornea 124 at three gazing positions:(1) gazing straight ahead and aligned with central axis 126, asindicated by cornea 124′ and corneal vector CV′; (2) gazing in theextreme positive direction, as indicated by cornea 124″ and cornealvector CV″; and (3) gazing in the extreme negative direction, asindicated by cornea 124′″ and corneal vector CV′″.

FIG. 2B depicts a schematic drawing of an exemplary scan region 122.Scan region 122 extends from x=xMin to x=xMax and from y=yMin to y=yMaxin the x- and y-directions, respectively.

In operation of system 100, scanner 202 sweeps input signal 116 overscan region 122 in two dimensions. When the input signal is incident oncornea 124, reflected signal 118 (i.e., the corneal reflection) sweepsover detector 204. It should be noted that the curvature of the corneagives rise to a reflective condition that reduces theangle-of-reflection to a narrow range of scanner angles. The position ofthe scanner that corresponds to the maximum received intensity at theaperture of detector 204 is then used to calculate the location of thecornea, which is then used to estimate corneal vector CV. A moredetailed discussion of system 100, its operation, and methods fortracking cornea 124 and estimating corneal vector CV is provided below.

A typical human eye has an eyeball diameter of approximately 24 mm and acornea having a 9 mm radius of curvature, with the cornea projecting afew millimeters above the eye, thereby defining a surface feature. Basedupon this typical eye configuration, in the depicted example, transmitmodule 102 and detect module 104 are positioned symmetrically aboutcentral axis 126 at half-width, W, (half the normal distance across atypical eyeglass lens) of approximately 25 mm. Vertex line 128 is astraight line connecting the center of scanner 202 and the center of theaperture of detector 204. Vertex distance, D, (i.e., the distancebetween vertex line 128 and the apex of cornea 124 when the eye isaligned with central axis 126) is selected as approximately 14 mm. Thelocations of transmit module 102 and detect module 104 are selected tosubstantially maximize the range over which a reflected signal 118 isreceived for all corneal locations within scan region 122.

As discussed below, an optional calibration procedure can be used todevelop a high-resolution relationship between scanner position,reflected beam position, and corneal location relative to a fixed pointin space. Such a calibration procedure can also compensate foruser-specific variations in system geometry, such as placement of theglasses on the face, corneal shape, etc. Such a calibration procedure ispreferably employed in cases where precise control of the absolute gazedirection is desired; however, for many applications, such as thosewhere relative gaze is tracked, calibration is not necessary.

It is another aspect of the present invention that the tracking functionof systems in accordance with the present invention relies only onintensity data observed at the fixed position of a discrete detector. Asa result, embodiments of the present invention do not require imagesensors and image processing algorithms, which are common to mostprior-art high-resolution eye-tracking systems. This reduces the powerconsumption, size, and cost of systems in accordance with the presentinvention, while simultaneously improving system bandwidth andresolution. Specifically, the resolution of embodiments of the presentinvention depends on the positioning resolution of the scanning deviceused, rather than the imaging resolution of a camera or intensityresolution of a position-sensitive detector.

Further, eye tracking using only a single scanned optical signal isfaster and consumes less power than the conventional methods, such asvideo-based eye-tracking. Detection circuits suitable for detecting asingle optical signal can also be significantly simpler than the typicalimage processing circuitry required in conventional, video-basedeye-tracking systems. In fact, such detection circuits can often beimplemented using conventional CMOS integration. In addition, the use ofa discrete detector avoids the need for image processing of a detectedimage to discern the location of the cornea. As a result, embodiments ofthe present invention can be much faster than video-based eye-trackingsystems of the prior art. Also, conventional video-based eye-trackingsystems are bulky and restrictive for the user due to the size andcomplexity of their required image sensors and processing systems. Incontrast, embodiments of the present invention can be simple and compactand, therefore, more unobtrusive to the user.

It should be noted that system 100 has better performance when cornea124 is rotated toward transmit module 102 relative to central axis 126(i.e., a is positive, as indicated in FIG. 2A) than when the cornea isrotated away from the transmit module (i.e., a is negative). This is dueto purely geometric considerations, since a given change in mirrorposition gives rise to a greater change in the angle at which reflectedsignal 118 reflects when a is positive than when it is negative, andalso since the angle of incidence onto the curved surface of the corneais more shallow when the cornea is pointing towards the detector. As aresult, in some embodiments, transmit and detect modules are located onboth sides of eye 120 to provide high-resolution tracking over theentire width of scan region 122. In other words, two systems 100(typically, with a single processor 106) are located on frames 108 butoriented in opposite directions.

In some embodiments, location sensitivity is improved by one or more of:

-   -   i. increasing vertex distance, D; or    -   ii. reducing half-width, W; or    -   iii. using a second system 100 to track the other eye of the        subject; or    -   iv. limiting the size of scan region 122; or    -   v. any combination of i, ii, iii, and iv.

It is yet another aspect of the present invention that the resolutioncriterion for determining the location of cornea 124 does not imposestringent constraints on the quality of the spot that is incident on thedetector of detect module 104. As discussed below, detect module 104includes a photodetector that provides an electrical signal thatrepresents the average optical power incident upon its aperture. Thiscan be written as a two dimensional integral of power over the surfacearea of the detector (at any position of the incidence beam). As thebeam sweeps the surface, the output of the detector may be calculated byan integral of the beam profile in the vertical direction, followed by aconvolution integral. It should be noted that these two integrals aredecoupled and therefore the profile of the beam is first integrated inthe vertical direction and then the convolution-like integral isevaluated:(p*g)(θ)=∫_(−∞) ^(∞) p(θ)g(θ−v)dv.  (1)

In Equation 1, the angular position, of scanner 202 shifts the spot g(θ)over the aperture p(θ) of detector 204 to perform convolution incylindrical coordinates. The integral is solved by multiplication in the(spatial) Fourier domain, which also shows that detector 204 performsspatial filtering on reflected signal 118. As a result, in order forsystem 100 to uniquely identify the location of cornea 124 that resultsin maximum reflection, the only requirement on the beam profile of inputsignal 116 is that it gives rise to a far-field pattern having a regionof global maximum intensity.

FIG. 3A depicts an example of a beam profile suitable for use in thepresent invention. Beam profile 300 is representative of the beamprofile of a low-cost vertical-cavity surface-emitting laser (VCSEL) andis characterized as a substantially circular beam having a“donut-shaped” intensity profile. In some embodiments, beam profile 300has a beam profile other than that of a donut.

FIG. 3B depicts a plot of the amplitude of the electrical signal outputby detector 204 as a function of mirror angle and percentage of the beamwaist of reflected signal 118 occupied by the aperture of detector 204.Plot 302 includes traces that correspond to percentages of beam waistoccupied by the aperture of detector 204 ranging from 1% to 10%, asindicated. It can be seen from plot 302 that, as long as detector 204has a sufficiently large aperture, its output signal will include asingle amplitude peak. In the depicted example, the spatial filteringprovided by detector 204 leads to the removal of any spatialirregularity in the detected beam waist when the aperture of detector204 occupies only 7% of the beam waist of reflected signal 118. In someembodiments, detector 204 includes a focusing lens for increasing itseffective aperture.

It should be noted that, typically, the spatial frequencies of theirregularities in the beam profile of even a low-cost VCSEL are highenough to allow the use of a detector having a relatively smallaperture, such as a surface-mount photodetector.

FIG. 4 depicts a schematic drawing of a perspective view of a transmitmodule in accordance with the illustrative embodiment. Transmit module102 includes scanner 202, which comprises source 402, fixed mirror 404,and scanning mirror 406. Scanner 102 is operative for scanning inputsignal 116 in two dimensions to interrogate the entirety of scan region122. In some embodiments, transmit module 102 does not interrogate theentirety of scan region 122.

Source 402 is a conventional VCSEL that is operatively coupled withsuitable drive circuitry (not shown). Source 402 generates input signal116 in response to command signal 128 from processor 106. As discussedabove, input signal 116 is preferably characterized by a beam profilethat gives rise to a far-field pattern having a global intensitymaximum. In some embodiments, source 402 is modulated to reduce powerconsumption. Modulation of source 402 also enables the signal-to-noiseratio (SNR) of system 100 to be increased.

Fixed mirror 404 is a conventional first-surface reflector suitable forreflecting the light output by source 402 toward scanning mirror 406without significant loss. Fixed mirror 404 may be used to align theoptical path in a manufacturing process.

Scanning mirror 406 is a two-dimensional MEMS-based scanning mirror thatis arranged to receive the light reflected toward it by fixed mirror 404and scan the light over scan region 122 as input signal 116.

In some cases, scanning mirror 406 is characterized by a curvature thatarises due to material stresses and features, such as etch releaseholes. In some embodiments, this curvature is exploited by controllingthe total path length between the emitting facet of source 402 and thereflecting surface of scanning mirror 406 such that the curvature of themirror compensates for the divergence of the light beam emitted from theVCSEL.

FIG. 5A depicts a schematic drawing of a scanning mirror in accordancewith the illustrative embodiment. Scanning mirror 406 includes mirror502, ϕ-actuator 504, θ-actuator 506, and anchor 508, which are disposedon substrate 510. Scanning mirror 206 is a MEMS-based, two-dimensionalscanning mirror suitable for fabrication via planar processingtechniques. Preferably, scanning mirror 406 is suitable for fabricationin a conventional CMOS foundry. Actuators suitable for use in scanningmirror 406, as well as methods suitable for forming them, are describedin U.S. Patent Publication 20150047078, entitled “Scanning ProbeMicroscope Comprising an Isothermal Actuator,” published Feb. 12, 2015,and U.S. Patent Publication 20070001248, entitled “MEMS Device HavingCompact Actuator,” published Jan. 4, 2007, each of which is incorporatedherein by reference.

Mirror 502 is a substantially square plate of single-crystal siliconthat is operative as a first-surface reflector for input signal 116. Insome embodiments, mirror 502 includes a surface layer of a highlyreflective material, such as gold, to enhance its reflectivity. Mirror502 is movable relative to substrate 510 and operatively coupled witheach of ϕ-actuator 504 and θ-actuator 506. In some embodiments, mirror502 comprises a different material suitable for use as a MEMS structuralmaterial, such as polysilicon, silicon carbide, silicon-germanium, aIII-V semiconductor, a II-VI semiconductor, a composite material, andthe like. In some embodiments, mirror 502 has a shape other than square,such as circular, elliptical, irregular, etc.

It is an aspect of the present invention that the use ofelectro-thermo-mechanical actuators to control the position of mirror502 about its rotation axes affords embodiments of the present inventionwith significant benefits, including:

-   -   i. CMOS-compatible operating voltage (3.3V); or    -   ii. small footprint (present embodiment 700 μm×700 μm); or    -   iii. large angular deflection (>45 degrees mechanical in 2 DOF);        or    -   iv. low power (<10 mW); or    -   v. high speed (≥5-kHz resonance); or    -   vi. low cost; or    -   vii. any combination of i, ii, iii, iv, v, and vi.        As a result, embodiments of the present invention preferably        employ electro-thermo-mechanical actuators for each of        ϕ-actuator 504 and θ-actuator 506. It is also preferred that        each of ϕ-actuator 504 and θ-actuator 506 is an isothermal        actuator since isothermal actuation mitigates parasitic effects        that arise from thermal coupling between axes of rotation. For        the purposes of this Specification, including the appended        claims, “isothermal operation” is defined as operation at a        constant power dissipation throughout an operating range. A        device or system that operates in isothermal fashion dissipates        constant power over its operating range, which results in a        steady-state heat flow into and out of the device or system. For        example, an isothermal actuator is an actuator that operates at        a constant power throughout its operating range. In some cases,        an isothermal actuator includes a plurality of actuating        elements where at least one of the actuating elements operations        in non-isothermal fashion; however, the plurality of actuating        elements are arranged such that they collectively operate in        isothermal fashion.

ϕ-actuator 504 is an isothermal torsional actuator operative forrotating plate 502 about the ϕ-axis, which is substantially aligned withthe x-axis in the depicted example. ϕ-actuator 504 includes torsionelements 512-1 and 512-2, each of which is mechanically coupled betweenmirror 502 and anchors 508 by beams 514. Beams 514 are rigid linkagescomprising the same structural material as mirror 502 (i.e.,single-crystal silicon).

Each of torsion elements 512-1 and 512-2 includes a plurality ofbimorphs 516, which are grouped into operative sets. Adjacent operativesets are rigidly interconnected via beams 514 such that bending of theoperative sets within a torsion element is additive. For clarity,elements comprising structural material (e.g., the material of mirror502, anchors 508, and beams 514) is depicted without cross-hatching,while bimorph elements 516 are depicted with cross-hatching.

Torsion elements 512-1 and 512-2 are rigidly connected rigid linkages518 and arranged such that they rotate about the ϕ-axis in the samedirection when subjected to opposite temperature changes. As a result,their collective power dissipation remains constant during operation.The temperature of torsion elements 512-1 and 512-2 is controlled viacontrolling electrical power dissipation (i.e., ohmic heating) in theelements themselves. In some embodiments, the temperature of thebimorphs in the torsional elements is controlled by controlling powerdissipation in ohmic heaters disposed on the elements. In someembodiments, a heat source external to the torsion elements is used tocontrol their temperature, such as heater elements disposed on thesurface of substrate 510.

θ-actuator 506 is an isothermal piston actuator operative for rotatingplate 502 about the θ-axis, which is substantially aligned with they-axis in the depicted example. θ-actuator 506 comprises piston elements520-1 through 520-4 (referred to, collectively, as piston elements 520)which are arranged in isothermal pairs. θ-actuator 506 is mechanicallycoupled between linkages 518 and anchors 508 by a set of beams 514. Eachof piston elements 520 includes a plurality of beams 514 and bimorphs516, which are arranged to give rise to vertical actuation in responseto a temperature change. The temperature of piston elements 520 iscontrolled as described above and with respect to torsional elements512.

Upon their release from substrate 510, piston elements 520 collectivelymove mirror 502 in the positive z-direction (i.e., away from thesubstrate surface). Each of the piston elements is designed such that anincrease in its power dissipation gives rise to its contraction, therebymoving its connection to mirror 502 toward the substrate. Pistonelements 520 are arranged in isothermal pairs—piston elements 520-1 and520-2 and piston elements 520-5 and 520-4. As a result, an increase inthe power dissipated in piston elements 520-2 and 520-5 and acommensurate decrease in the power dissipated in piston elements 520-1and 520-4 induces positive (as indicated) rotation of mirror 502 aboutthe θ-axis while maintaining a constant power dissipation in θ-actuator506 overall. In similar fashion, by decreasing the power dissipated inpiston elements 520-2 and 520-5 and increasing the power dissipated inpiston elements 520-1 and 520-4 by the same amount, a negative rotationof mirror 502 about the θ-axis is induced while the power dissipated inθ-actuator 506 remains constant.

It should be noted that the actuator and mirror configuration ofscanning mirror 406 is one of many possible MEMS-based scanning mirrorconfigurations within the scope of the present invention. Somealternative embodiments in accordance with the present invention includea ϕ-actuator and/or θ-actuator that is actuated by another actuationmeans, such as electrostatic, electromagnetic, magnetostrictive,piezoelectric, and the like. Some alternative embodiments in accordancewith the present invention include an ϕ-actuator and/or θ-actuator thatis non-isothermal. Some alternative embodiments in accordance with thepresent invention include a movable mirror that includes an opticalelement, such as one or more diffractive lenses (e.g., a one- ortwo-dimensional Fresnel lens, a holographic lens, etc.), one or morerefractive lenses, an active optical source, one or more diffractiongratings, one or more prisms, and the like.

FIG. 5B depicts a photograph of a scanning mirror analogous to scanningmirror 406; however, scanning mirror 522 includes a non-isothermaltorsional actuator, actuator 524, for rotation about the ϕ-axis and anisothermal piston actuator 506 for rotation about the θ-axis, asindicated.

FIG. 5C depicts an electrical arrangement suitable for driving ascanning mirror that employs isothermal actuators for each axis ofrotation. Circuit 526 includes source 528 and terminals 530-1 and 530-2.Terminal 530-1 receives an ϕ-axis control signal from processor 106 thatalters the current flow through torsion elements 512-1 and 512-2,thereby determining their relative power dissipation. In similarfashion, terminal 530-2 receives a θ-axis control signal from processor106 that alters the current flow through piston elements 520-1 through520-4, thereby determining their relative power dissipation.

The arrangement of circuit 526 reduces the number of drive signalsrequired in control signal 128 by a factor of 2, thereby reducing thecost and complexity of the drive electronics included in transmit module102.

It should be noted that, preferably, PWM signals are used at terminals530-1 and 530-2. The use of PWM signals enables linear control of thepower dissipated by the resistance of each electro-thermo-mechanicalelement while the overall power dissipated in each axis remainsconstant.

FIGS. 6A-D depict photographs of alternative MEMS-based scanning mirrorsin accordance with the present invention. Mirrors 600, 606, 612, and 618show a variety of different combinations of actuators andoptical-element-containing mirrors. It will be clear to one skilled inthe art, after reading this Specification, however, that mirrors 600,606, 612, and 618 represent merely a few of the possible mirror andactuator combinations within the scope of the present invention.

FIG. 6A depicts a photograph of an alternative scanning mirror inaccordance with the present invention. Scanning mirror 600 includesmirror 502, isothermal piston actuators 520-1 and 520-2 for mirrorrotation about the θ-axis, and non-isothermal resonant bimorph actuator602 for inducing mirror rotation about the ϕ-axis.

Bimorph actuator 602 includes vertical actuators 604-1 and 604-2. Eachvertical actuator includes a set of bimorphs 516, which are arranged tobend away from substrate 510 upon their release during MEMS fabrication.During operation, bimorphs 516 bend downward (toward substrate 510) inresponse to an increase in their temperature and upward in response to adecrease in temperature. By applying a signal at the natural frequencyof the device, the first resonant mode causes actuators 604-1 and 604-2to rotate the mirror 502 about the ϕ-axis. The isothermal nature of theθ-axis actuator ensures that coupling between the orthogonal axes issuppressed.

FIG. 6B depicts a photograph of another alternative scanning mirror inaccordance with the present invention. Scanning mirror 606 is analogousto scanning mirror 600; however, scanning mirror 606 includes circularmirror 608, which includes integrated optical element 610. In thedepicted example, optical element 610 is a Fresnel lens for forming asubstantially circular spot on eye 120; however, one skilled in the artwill recognize, after reading this Specification, that optical element610 can include any conventional diffractive or refractive elementsuitable for fabrication in, or disposition on, mirror 608 (or anymirror in accordance with the present invention). Optical elementssuitable for use in embodiments of the present invention include,without limitation, refractive lenses, diffractive lenses, holographiclenses, and the like.

FIG. 6C depicts a photograph of another alternative scanning mirror inaccordance with the present invention. Scanning mirror 612 is analogousto scanning mirror 600; however, scanning mirror 612 includes opticalelement 616, which is integrated into the surface of mirror 502.

In the depicted example, optical element 616 includes a pair ofcylindrical Fresnel lenses that are arranged orthogonally and formed inthe body of mirror 502. As a result, mirror 502 forms input signal 116into a cross-shaped light pattern on scan region 122. The use of aninput signal that forms a cross pattern on the eye can advantageouslyimprove the robustness of tracking algorithms used to track the locationof cornea 124 within scan region 122. An exemplary cross-shaped lightpattern 618, formed by optical element 616, is depicted in the inset inthe upper right corner of FIG. 6C.

FIG. 6D depicts a photograph of yet another alternative scanning mirrorin accordance with the present invention. Scanning mirror 620 isanalogous to scanning mirror 606; however, scanning mirror 620 includesisothermal torsional actuators 504-1 and 504-2 for rotating mirror 608about the θ- and ϕ-axes, respectively.

Embodiments of the present invention preferably employelectro-thermo-mechanical actuators, such as those described above, tocontrol the position of mirror 502 about its rotation axes. Suchactuators offer unparalleled positioning accuracy and repeatability inresponse to applied drive signals. As a result, they can be operated inopen-loop fashion without incurring significant positioning inaccuracy.In addition, by employing structural material that remains in a linearstress-strain configuration throughout the operating range of theactuator, little or no drift in position response arises over time. Insome embodiments, however, sensors (e.g., piezoresistive strain sensors,capacitive sensors, optical sensors, etc.) are included in the scanningmirror to further improve positioning accuracy. Further, in someembodiments, a pulse-width modulation (PWM) drive scheme is employed tocontrol the position of the scanning mirror, as discussed below.

In some embodiments, piezoresistive strain sensors are included in thebeams of the thermal actuators to enable the devices to be placed inresonance so as to maintain constant phase and amplitude over a widerange of operating conditions.

In some embodiments, scanning mirror 406 is another scanning elementsuitable for scanning input signal 116 over scan region 122. Scanningelements suitable for use in the present invention include, withoutlimitation, scanning prisms, Risley prisms, tunable diffractiongratings, variable blazed gratings, coupled pairs of single-axisscanning mirrors, non-MEMS-based scanning mirrors, electro-optic beamscanners, galvanometer mirrors, and the like.

In some embodiments, the output of source 402 is provided directly toscanning mirror 406. In some embodiments, source 402 is opticallycoupled with scanning mirror 406 via a waveguide such as an opticalfiber, planar-lightwave circuit waveguide, and the like.

In some embodiments, transmit module 102 includes a scanner wherein theorientation of source 402, itself, is scanned in one or two dimensionsrelative to scan region 122. In other words, such a scanner combines theoperations of both source 402 and scanning mirror 406 in a singleintegrated device. For example, in some embodiments, an active lightsource is integrated directly onto the surface of scanning mirror 406via hybrid and/or monolithic integration techniques. Monolithicintegration techniques suitable for integrating an active light sourceon scanning mirror 406 include, for example, those described by Liang,et al., in “Hybrid Integrated Platforms for Silicon Photonics,”Materials, Vol. 3, pp. 1782-1802 (2010), which is incorporated herein byreference.

Returning now to the illustrative embodiment, FIG. 7 depicts a schematicdrawing of a detect module in accordance with the illustrativeembodiment. Detect module 104 includes detector 204 and detectioncircuit 702.

As discussed above, detector 204 is a conventional,single-detection-region photodetector for receiving reflected signal 118and providing a single electrical signal, electrical signal 704, suchthat the magnitude of electrical signal 704 is based on the intensity ofreflected signal 118. Detector 204 has sensitivity and detectionbandwidth sufficient to enable eye tracking at a rate dictated by theapplication for which system 100 is intended. Typically, detector 204enables operating rates within the range of a few Hz to tens of kHz.Detector 204 has a detection surface that is large enough to accommodatethe full beam profile of reflected signal 118 such that detector 204 canprovide an output that represents the average power of the lightincident upon it. Typically, detector 204 has a substantially squaredetection surface that is tens to hundreds of microns on a side. In thedepicted example, detector 204 is a photodetector having a squaredetection region of 500 microns on a side. In some embodiments, detector204 includes a filter for blocking ambient light from reaching itsaperture. In some embodiments, detector 204 includes a focusing lens forincreasing the effective aperture of the photodetector. One skilled inthe art will recognize, after reading this Specification, that detector204 can include any optoelectronic device operative for converting anoptical signal into an electrical signal, wherein the magnitude of theelectrical signal is based on the intensity of the light signal.

Detection circuit 702 is an integrated circuit operative for identifyingpeaks in electrical signal 704 and providing output signal 126 toprocessor 106. In the depicted example, detection circuit 702 includes apeak-and-hold circuit 706 to set a threshold value for a comparator 710,which collectively convert eye position into a time-domain signal thatis readily captured by processor 106. The operation of detection circuit702 is described in detail below and with respect to FIGS. 9 and 10. Oneskilled in the art will recognize, after reading this Specification,that myriad alternative detection circuits are within the scope of thepresent invention.

In some embodiments, detector 204 is a discrete detector that includesmore than one detection region and, therefore, provides more than oneelectrical output signal. In such embodiments, detection circuit 702 isoperative for determining the relative magnitudes of the plurality ofelectrical signals. These relative magnitudes can then be used toestimate an offset of the position at which a reflection maximum isdetected during a scan of input signal 116 through scan region 122 fromthe location of the global reflection maximum within the complete areaof scan region 122. As a result, the paths of subsequent scans can beadjusted to more quickly intersect the location of the global maximumand identify the location of cornea 124 and corneal vector CV.

It should be noted that, within the scope of the present invention,there are numerous approaches for scanning input signal within scanregion 122 that are effective for identifying the two-dimensionallocation of a surface feature within scan region 122. Severalnon-limiting examples are provided herein.

FIG. 8 depicts an initialization procedure for approximating thelocation of a surface feature on an eye. Method 800 is typicallyperformed prior to any of the high-resolution eye tracking methodsdescribed below in order to establish an initial point, IP, at which thesubsequent higher-resolution method should begin. Method 800 begins withoperation 801, wherein scan region 122 is divided into an array of Mpaths, P(1) through P(M), each of which traverses the scan region alonga first direction, and where the paths are equally arranged along asecond direction that is orthogonal to the first direction. In thedepicted example, M=8 and each path, P(j), where i=1 through 8, isaligned with the x-direction. The 8 paths are equally spaced along they-direction from y=yMin to yMax. It should be noted that the value of M,as well as the spacing between the paths, is a matter of design choiceand is typically based upon the size of input signal 116, the size ofscan region 122, and the desired operating rate of system 100.

For j=1 through M:

At operation 802, input signal 116 is swept back and forth along path,P(i).

At operation 803, first and second peaks, p1(j) and p2(j) in electricalsignal 704 are identified, as are times t1(j) and t2(j) at which theyare detected.

At operation 804, a first point of maximum reflectivity, RM(j) in pathP(j) is determined based on times t1(j) and t2(j).

At operation 805, a second point of maximum reflectivity along they-direction is established based on the magnitudes of p1(1) throughp1(8). In some embodiments, the second point is established as they-position of the path having the largest peak. In some embodiments, thesecond point is based on the complete set of the magnitudes of peaksp1(1) through p1(8) using, for example, interpolation, etc.

At operation 806, the initial position, IP, is established based on thefirst and second points.

Method 800 is preferably performed prior to employing subsequenthigher-resolution eye-tracking methods in accordance with the presentinvention (examples of which are described below); however, in someembodiments, natural eye movement and motion of scanner 202 due toelectrical noise is relied upon to give rise to sufficient intersectionof input beam and cornea 124 to establish the initial position.

As mentioned briefly above, in applications where high-resolutionabsolute eye tracking is desired, an optional calibration routine can beperformed after the initialization procedure outlined above. Calibrationimproves the accuracy of the relationship between the mirror 502 aboutthe ϕ- and θ-axes and the direction of the user's gaze (i.e., cornealvector, CV). In a typical calibration procedure, the test subject isinstructed to look at a plurality of distinct points (e.g. nine) on ascreen. The position of mirror 502 that corresponds to the peakintensity at each point is then measured. A coordinate transformationbetween the measured positions to the actual positions is calculated.This coordinate transformation is then applied to subsequenteye-tracking measurements. It should be noted that system 100 remainscalibrated even after removal of the screen as long as the relativepositions of scanner 202, detector 204, and eye 120 remain fixed.

An alternative calibration procedure that requires significantly lesscomputation can also be used in many instances. In this alternativeprocedure, the test subject presses a “grab” button while looking at anarbitrary point on the screen. The point measured by the eye-trackingsystem is then displayed on the screen. Typically, initially, themeasured point is slightly offset from the point at which the user islooking. While the grab button is pressed, the test subject redirectstheir gaze at the projected point. Upon release of the grab button, theoffset between the two points at which the test subject's eye had beendirected is calculated and the difference is subtracted from themeasured value. Repeated performance of this alternative calibrationprocedure results in additional improvement in the calibration.

FIG. 9 depicts operations of a method suitable for tracking the positionof a surface feature of an eye in accordance with the illustrativeembodiment of the present invention. Method 900 begins with operation901, wherein clock reset signal R1 is provided to transmit module 102and detect module 104 on control signal 128. Clock reset signal R1restarts a system clock within processor 106 at time t0 and signals totransmit module 102 that it should direct input beam to an initialposition on a first path through scan region 122.

FIG. 10 depicts an exemplary timing diagram in accordance with method900. Plot 1000 shows: (1) the position of input signal 116 along each ofthe x- and y-directions within scan region 122; (2) the magnitude ofelectrical signal 704; and (3) the magnitude of output signal 126 (i.e.,the output of a comparator included in detection circuit 704)—each as afunction of time, t.

At operation 902, input signal 116 is swept at a constant speed back andforth along a first path. Typically, the first path is located along they-direction such that it includes the initial point, as discussed above.In the depicted example, the first path is a linear path aligned withthe x-axis and substantially centered on the y-axis (i.e., midwaybetween yMin and yMax as indicated in FIG. 2B). In some embodiments, thefirst path is another path through scan region 122. In some embodiments,the first path is linear path that is unaligned with either of the x-and y-axes.

At operation 903, first and second maxima (i.e., peaks) in electricalsignal 704 are identified. As indicated in plot 1000, the first peakoccurs at time t1, which arises during the first half of period Tx asinput signal 116 travels in the positive x-direction. In similarfashion, the second peak occurs at time t2, which arises during thesecond half of period Tx as input signal 116 travels in the negativex-direction. The speed at which input beam 116 is swept across scanregion 122 is preferably higher than the speed at which eye 120 canmove. As a result, the first and second peaks correspond to the samecorneal location within the scan region.

In the example depicted in plot 1000, times t1 and t2 correspond to thepoint at which the rising edge of electrical signal 704 exceedscomparator threshold 1002. In some embodiments, another feature ofelectrical signal 704 is used to determine times t1 and t2, such as thepoint at which the falling edge of electrical signal 704 decreases belowcomparator threshold 1002, or the midpoint between the points at whichthe rising and falling edges cross threshold 1002, etc. Typically, thedetection of the timing of an edge of a pulse is less susceptible tonoise than detecting the positions of the apex of a peak. Further, theresolution of detection circuit 702 improves as clock speed isincreased. Still further, additional improvement in system SNR can begained by modulating input signal 116 and demodulating electrical signal704.

At operation 904, a first point of maximum reflectivity on the firstpath is estimated based on the time interval between times t1 and t2.Since input signal 116 is swept at a constant speed, the time intervalbetween times t1 and t2 indicates the distance the input signal hastraveled between peaks and, therefore, can be used to determine thepoint of maximum reflectivity along the first path.

At operation 905, clock reset signal R2 is provided to transmit module102 and detect module 104. Clock reset signal R2 restarts the systemclock at time t0 and signals to transmit module 102 that it shoulddirect input beam to an initial position on a second path through scanregion 122, where the second path includes the first point of maximumreflectivity on the first path. Typically, although not necessarily, thesecond path is a linear path that is orthogonal to the first path. Inthe depicted example, therefore, the second path is aligned with they-axis.

At operation 906, input signal 116 is swept at constant speed back andforth along the second path between yMin to yMax.

At operation 907, third and fourth peaks in electrical signal 704 areidentified. As indicated in plot 1000, the third peak occurs at time t3,which arises during the first half of period Ty as input signal 116travels in the positive y-direction. In similar fashion, the fourth peakoccurs at time t4, which arises during the second half of period Ty asinput signal 116 travels in the negative y-direction.

At operation 908, a second point of maximum reflectivity on the secondpath is estimated based on the time interval between times t3 and t4.Since input signal 116 is swept at a constant speed in the y-direction,the time interval between times t3 and t4 indicates the distance theinput signal has traveled between maxima and, therefore, the point ofmaximum reflectivity along the second path.

At operation 909, a two-dimensional location of cornea 124 within scanregion 122 is estimated based on the points of maximum reflectivity onthe first and second paths.

By continuously repeating method 900, the location of cornea 124 withinscan region 122 can be constantly tracked.

Although mirror 502 is scanned at constant speed in the illustrativeembodiment, it will be clear to one skilled in the art, after readingthis Specification, how to specify, make, and use alternativeembodiments of the present invention wherein mirror 502 is scanned usingan alternative scanning method. Furthermore, in some embodiments,position is estimated by methods other than detecting the timing ofpulses. For example, in some embodiments, a sinusoidal signal is used toexcite the scanner at high speeds, while peak positions are estimatedwith a sin⁻¹ factor. In other embodiments, the actual position of mirror502 at which a maximum occurs is used directly based on the voltagesprovided by processor 106 in control signal 128. In some embodiments,the position of mirror 502 is detected via a sensor that is operativelycoupled with scanner 202.

It should be noted that method 900 has the disadvantage that abruptchanges in the micromirror position can result in mechanical ringing inthe position of mirror 502. Such ringing can give rise to a limit on thespeed and resolution that may be obtained with system 100.

FIG. 11 depicts operations of a second exemplary method suitable fortracking the position of a surface feature of an eye in accordance withthe present invention. Method 1100 begins with operation 1101, wherein aglobal clock reset signal is provided to transmit module 102. The globalclock reset signal indicates the beginning of a raster scan periodduring which the entirety of scan region 122 is to be interrogated.

At operation 1102, input signal 116 is scanned at a constant rate ofspeed along a raster-scan path through scan region 122. In someembodiments, the scan rate along the x-direction is different than thescan rate along the y-direction.

In some embodiments, a different two-dimensional scanning path is usedin operation 1102. Scanning paths suitable for use in embodiments of thepresent invention include, without limitation, Lissajous patterns,rhodonea curves, circular paths, elliptical paths, and the like. Itshould be noted that, when using continuous two-dimensional pathsthrough scan region 122 (e.g., Lissajous patterns and rhodonea curves),the timing of the maxima in electrical signal 704 reveals informationabout the x and y position of cornea 124. In some embodiments, thetrajectory of mirror 502 is offset in order to maintain a constant phaserelationship between peaks. In some embodiments, the phase differencebetween the steady-state signals applied to each axis is adjusted to“sweep” the two-dimensional pattern over the eye. A drawback oftwo-dimensional scanning of scan region 122 is a reduction in thebandwidth of system 100.

As discussed above, in some embodiments, input signal 116 is formed as acrosshair pattern on eye 120. This is particularly advantageous fortwo-dimensional scanning approaches because such a light patternvirtually ensures that input signal 116 will traverse the point ofmaximum reflectivity.

At operation 1103, a plurality of maxima (i.e., peaks) in electricalsignal 704, and their corresponding detection times, are identified.

At operation 1104, the location of input beam 116 corresponding to thepeak having the largest magnitude is determined.

In some embodiments, the location of input beam 116 corresponding to thepeak having the largest magnitude is used to estimate the location ofcornea 124 in the y-direction, while the timing between adjacent peaksin at least one x-direction scan portion is used to estimate thelocation of cornea 124 in the x-direction.

FIG. 12 depicts operations of a third exemplary method suitable fortracking the position of a surface feature of an eye in accordance withthe present invention. Method 1100 employs the back-and-forth scanningapproach to identify the point of maximum reflection in a firstdimension, while simultaneously using a feedback control system toidentify the point of maximum reflectivity in a second dimension that isorthogonal to the first dimension. In the depicted example, the firstdimension is the x-direction and the second dimension is they-direction. For the purposes of this Specification, including theappended claims, a “feedback control system” is defined as system ofelements or commands that manages the operation of an eye-trackingsystem based on one or more measurements of the system output over time.Examples of feedback control systems in accordance with this definitioninclude proportional-integral-derivative controllers (PID controllers),for proportional-summation-difference controllers (PSD controllers),hill-climbing controllers, Kalman Filters, and the like.

Method 1200 begins with operation 1201, wherein input signal 116 isswept at a constant speed back and forth along a first path, P(1).Typically, the P(1) is located along the y-direction such that itincludes the initial point, as discussed above. In the depicted example,the first path is a linear path aligned with the x-axis and includes theinitial point, IP, as discussed above. In some embodiments, the firstpath is another path through scan region 122. In some embodiments, thefirst path is linear path that is unaligned with either of the x- andy-axes.

At operation 1202, first and second peaks, p1(1) and p2(1) in electricalsignal 704 are identified, as are times t1(1) and t2(1) at which theyare detected.

At operation 1203, the point of maximum reflectivity, RM(1), in pathP(1) is determined based on times t1(1) and t2(1).

At operation 1204, the magnitude of peak p1(1) is stored in memory. Notethat the magnitudes of the peaks within a single scan line aresubstantially identical, as a result, the choice of which of peaks p1(1)and p2(1) to store is arbitrary.

For 1=2 through N, where N is an arbitrary number:

At operation 1205, wherein input signal 116 is swept at a constant speedback and forth along a first path, P(i), where P(i) is offset from P(1)along the y-direction by an arbitrary distance.

At operation 1206, first and second peaks, p1(i) and p2(i) in electricalsignal 704 are identified, as are times t1(i) and t2(i) at which theyare detected.

At operation 1207, the point of maximum reflectivity in path P(i) isdetermined based on times t1(i) and t2(i).

At operation 1208, the magnitude of peak p1(i) is stored in memory.

At operation 1209, A second point of maximum reflectivity in they-direction is determined by comparing RM(i) and RM(i−1) via a feedbackcontrol system.

At operation 1210, a location of cornea 124 within scan region 122 isdetermined based on the first point and second point.

It should be noted that, while the embodiments described herein aredirected toward eye-tracking applications, the present invention can bedirected at many alternative applications, such as head tracking,virtual keyboards based on finger tracking, and free-spacecommunications links, among others.

In an exemplary head-tracking system in accordance with the presentinvention, a beam of light is directed from the head of a test subjecttoward a display (or other object). A small retro-reflector placed onthe object reflects the beam of light back toward a scanner (e.g.,scanner 202). Detector 204 is located adjacent to scanner 202 such thatit can detect the reflected light. A timing circuit is used to estimatethe orientation of the test subject's head with respect to the object.By including a second retroreflector, both the position and orientationof the test subject's head can be estimated.

A virtual keyboard in which the position of one or more fingers incontact with an arbitrary flat surface is also enabled by the presentinvention. In an exemplary virtual keyboard system, the surface isscanned with a narrow interrogation signal at a glancing angle. As aresult, light reflected and scattered from the surface is not directedback towards the source. A photodetector is aligned flush with thesurface, and an aperture restricts light that is incident on thephotodetector. Only light that is directed substantially along thesurface is detected, therefore. By performing a raster scan of thesurface, the two-dimensional position of the one or more fingers incontact with the surface can be estimated.

The present invention also enables a free-space optical communicationslink comprising at least two nodes, where each node comprises at leastone scanner, one retroreflector and one detector. A data-modulated beamof light directed from a first node towards a second node is directedback toward the detector of the first node by the retroreflector on thesecond node. Translation of the second node (for example, if the node islocated on top of a building that sways due to wind or other forces)would normally reduce the intensity of the reflected beam. In accordancewith the present invention, however, the scanner in the first node cantrack the position of the second node by monitoring the reflected powervia its detector. Once the link is established, network traffic may bemodulated onto the free-space optical link. An ad-hoc network can beformed with an arbitrary number of nodes, therefore, and can adapt tothe addition or removal of nodes. When a two-way link is formed betweentwo nodes, the power from one incoming beam can be transmitted as data(overhead) through the link and used for robust mutual tracking.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

What is claimed is:
 1. A system for estimating the corneal vector of thecornea of an eye, the system comprising: a first source that provides afirst optical signal; a first scanner that scans the first opticalsignal within a scan region on the surface of the eye, the scan regionincluding the cornea; a first detector that is a discrete detector thatprovides a first electrical signal that is based only on the intensityof light of the first optical signal reflected from the scan region tothe first detector, wherein the light reflected from the scan regiondoes not form an image of a portion of the eye on the first detector,and wherein the first electrical signal excludes spatially correlatedimage information; and a detection circuit that determines at least onemaximum in the first electrical signal.
 2. The system of claim 1 furthercomprising: a second source that provides a second optical signal; asecond scanner that scans the second optical signal within the scanregion; and a second detector that is a discrete detector, wherein thesecond detector provides a second electrical signal based only on theintensity of light of the second optical signal reflected from the scanregion to the second detector; wherein the detection circuit alsodetermines at least one maximum in the second electrical signal; andwherein the first scanner, second scanner, first detector, and seconddetector are arranged such that the first scanner and the seconddetector are located on a first side of the scan region and the secondscanner and the first detector are located on a second side of the scanregion that is opposite the first side.
 3. The system of claim 1 furthercomprising a processor operative for (1) estimating a location of thesurface feature based on the at least one maximum and (2) estimating thecorneal vector based on the location.
 4. The system of claim 1 furthercomprising an optical element that projects the first optical signal onthe eye as a first light pattern.
 5. The system of claim 4 wherein thefirst light pattern is selected from a group consisting of a spot and across.
 6. The system of claim 4 wherein the scanner includes the opticalelement.
 7. The system of claim 1 wherein the scanner includes thesource.
 8. The system of claim 1 wherein the scanner comprises: areflector that defines a first plane; and a two-dimensional actuatorthat controls the angle of the first plane relative to each of a firstaxis and a second axis.
 9. The system of claim 8 wherein thetwo-dimensional actuator includes a first isothermal actuator that isoperative for rotating the reflector about the first axis.
 10. Thesystem of claim 1 wherein the first detector comprises more than onedetection region.
 11. The system of claim 1 wherein the first opticalsignal is characterized by a far-field pattern having a global intensitymaximum.
 12. The system of claim 1 wherein the second optical signal hasa first beam waist at the first detector, and wherein the first detectoris characterized by a first aperture, and further wherein the relativesizes of the first aperture and first beam waist give rise to a singleamplitude peak in the first electrical signal when the second lightsignal is at least partially incident on the first detector.
 13. Asystem for estimating the corneal vector of the cornea of an eye withoutforming an image of a portion of the eye, the system comprising: a firstscanning mirror that scans a first optical signal within a scan regionon the surface of the eye, wherein the scan region includes the cornea;a first detector that is a discrete detector, wherein the first detectorreceives a reflection of the first light signal from the scan region asa second optical signal, and wherein the first detector provides a firstelectrical signal based solely on the second optical signal, and furtherwherein the first electrical signal excludes spatially correlated imageinformation; a detection circuit that determines a first maximum in thefirst electrical signal at a first time and a second maximum in thefirst electrical signal at a second time; and a processor that (1)estimates the location of a surface feature within the scan region basedon the difference between the first time and second time and (2)estimates the corneal vector based on the location.
 14. The system ofclaim 13 wherein the first source and first scanning mirror are affixedto a frame such that they are located on a first side of the scanregion, and wherein the first detector is affixed to the frame such thatit is located on a second side of the scan region, the scan region beingbetween the first and second sides.
 15. The system of claim 13 whereinthe first scanning mirror is dimensioned and arranged to project thefirst optical signal onto the eye as a first light pattern.
 16. Thesystem of claim 15 wherein the first light pattern is selected from agroup consisting of a spot and a cross.
 17. The system of claim 13further comprising: a second source that provides a third opticalsignal; a second scanning mirror that scans the third optical signalwithin the scan region; a second detector that is a discrete detector,wherein the second detector receives a reflection of the third opticalsignal from at least a portion of the scan region as a fourth opticalsignal, and wherein the second detector provides a second electricalsignal based solely on the fourth optical signal; wherein the detectioncircuit determines a third maximum in the second electrical signal at athird time and a fourth maximum in the second electrical signal at afourth time; and wherein the processor (3) further estimates thelocation of the surface feature within the scan region based on thedifference between the third time and fourth time.
 18. The system ofclaim 13 wherein the first scanning mirror includes: a substrate; amirror that is reflective for the first optical signal, the mirror beingmovable relative to the substrate, and the mirror defining a first planehaving a first and second axes that are orthogonal; a first actuatoroperative for rotating the mirror about the first axis; and a secondactuator operative for rotating the mirror about a fourth axis that issubstantially parallel with the second axis; wherein at least one of thefirst actuator and second actuator is an isothermal actuator.
 19. Thesystem of claim 18 wherein the first actuator is an isothermal actuatorthat includes: a first torsional element comprising a first plurality ofbimorphs whose curvature is based on a first temperature; and a secondtorsional element comprising a second plurality of bimorphs whosecurvature is based on a second temperature; wherein the substrate,mirror, first torsional element, and second torsional element arearranged such that (1) a first rotation of the mirror about the firstaxis is induced by an increase in the first temperature and a decreasein the second temperature and (2) a second rotation of the mirror aboutthe first axis is induced by a decrease in the first temperature and anincrease in the second temperature, the first and second rotations beingopposite rotations.
 20. The system of claim 18 wherein each of the firstactuator and second actuator is an isothermal actuator.
 21. The systemof claim 18 wherein one of the first actuator and second actuator is anon-isothermal actuator.
 22. The system of claim 13 wherein the firstdetector comprises more than one detection region.
 23. The system ofclaim 13 wherein the first optical signal is characterized by afar-field pattern having a global intensity maximum.
 24. The system ofclaim 13 wherein the second optical signal has a first beam waist at thefirst detector, and wherein the first detector is characterized by afirst aperture, and further wherein the relative sizes of the firstaperture and first beam waist give rise to a single amplitude peak inthe first electrical signal when the second light signal is at leastpartially incident on the first detector.
 25. A method for estimatingthe corneal vector of the cornea of an eye, the method comprising:scanning a first optical signal within a scan region on the surface ofthe eye, wherein the scan region includes the cornea; reflecting atleast a portion of the first optical signal from the cornea as a secondoptical signal; receiving the second optical signal at a first detectorthat is a discrete detector; generating a first electrical signal basedsolely on the second optical signal, the first electrical signal beinggenerated by the first detector, wherein the first electrical signalexcludes spatially correlated image information; determining at leastone maximum of the second optical signal; estimating a first location ofa surface feature within the scan region based on the at least onemaximum; and estimating the corneal vector based on the first location.26. The method of claim 25 further comprising providing the firstoptical signal as a first light pattern on the eye.
 27. The method ofclaim 25 wherein the first optical signal is scanned over the surface byoperations including: generating the first optical signal at a source;reflecting the first optical signal from a scanner toward the scanregion; and controlling a rotation of the scanner about at least oneaxis of rotation.
 28. The method of claim 25 wherein the first opticalsignal is scanned over the surface by operations including: generatingthe first optical signal at a source; and controlling a rotation of thesource about at least one axis of rotation.
 29. The method of claim 25wherein the first optical signal is scanned over the surface byoperations comprising: scanning the first optical signal within the scanregion such that it traverses back and forth along a first path that islinear along a first axis during a first time period; determining afirst maximum in the first electrical signal at a first time and asecond maximum in the first electrical signal at a second time;estimating a first point of maximum reflection on the first axis basedon a difference between the first time and second time; scanning thefirst optical signal such that it traverses back and forth along asecond path that is linear along a second axis during a second timeperiod, wherein the second path includes the first point, and whereinthe first and second axes are orthogonal; determining a third maximum inthe first optical signal at a third time and a fourth maximum in thefirst optical signal at a fourth time; and estimating a second point ofmaximum reflection on the second axis based on the difference betweenthe third time and fourth time; estimating the first location based onthe first point and the second point; and estimating the corneal vectorbased on the first location.
 30. The method of claim 25 wherein thefirst optical signal is scanned over the surface by operationscomprising: repeatedly sweeping the first optical signal such that ittraverses back and forth along a first linear path along a first axis;for each sweep, determining a first maximum of the second optical signalat a first time and a second maximum of the second optical signal at asecond time; for each sweep, estimating a second location along thefirst axis based on a difference between the first time and second time;moving the location of the first linear path in a first direction alonga second axis that is substantially orthogonal to the first axis;measuring a change in the magnitude of the first electrical signal; andmoving the location of the first linear path (1) further along the firstdirection when the change is positive and (2) in a second directionalong the second axis when the change is negative, wherein the firstdirection and second direction are opposite directions on the secondaxis.
 31. The method of claim 25 wherein the first optical signal isscanned within the scan region along a two-dimensional path selectedfrom the group consisting of Lissajous curves, rhodonea curves,elliptical paths, circular paths, and raster-scan paths.